Intervertebral spacer device having a multi-pronged domed spring

ABSTRACT

An intervertebral spacer device having a pair of opposing plates for seating against opposing vertebral bone surfaces, separated a spring mechanism. The preferred spring mechanism is a multi-pronged domed spring which is coupled to the upper plate by set screws. The spring includes a socket formed in the peak thereof and mounts onto a ball-shaped head extending outwardly from the lower plate. The spring and post members are thereby flexibly coupled such that the upper and lower plates may rotate relative to one another.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation in part of U.S. patentapplication Ser. No. 09/982,148 filed Dec. 18, 2001 and entitled “AnIntervertebral Spacer Device Having Arch Shaped Spring Elements”, whichis fully incorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates generally to a spinal implant assembly forimplantation into the intervertebral space between adjacent vertebralbones to simultaneously provide stabilization and continued flexibilityand proper anatomical motion.

BACKGROUND OF THE INVENTION

[0003] The bones and connective tissue of an adult human spinal columnconsists of more than 20 discrete bones coupled sequentially to oneanother by a tri-joint complex which consists of an anterior disc andthe two posterior facet joints, the anterior discs of adjacent bonesbeing cushioned by cartilage spacers referred to as intervertebraldiscs. These more than 20 bones are anatomically categorized as beingmembers of one of four classifications: cervical, thoracic, lumbar, orsacral. The cervical portion of the spine, which comprises the top ofthe spine, up to the base of the skull, includes the first 7 vertebrae.The intermediate 12 bones are the thoracic vertebrae, and connect to thelower spine comprising the 5 lumbar vertebrae. The base of the spine isthe sacral bones (including the coccyx). The component bones of thecervical spine are generally smaller than those of the thoracic spine,which are in turn smaller than those of the lumbar region. The sacralregion connects laterally to the pelvis. While the sacral region is anintegral part of the spine, for the purposes of fusion surgeries and forthis disclosure, the word spine shall refer only to the cervical,thoracic, and lumbar regions.

[0004] The spinal column of bones is highly complex in that it includesover twenty bones coupled to one another, housing and protectingcritical elements of the nervous system having innumerable peripheralnerves and circulatory bodies in close proximity. In spite of thesecomplications, the spine is a highly flexible structure, capable of ahigh degree of curvature and twist in nearly every direction.

[0005] Genetic or developmental irregularities, trauma, chronic stress,tumors, and degenerative wear are a few of the causes which can resultin spinal pathologies for which surgical intervention may be necessary.A variety of systems have been disclosed in the art which achieveimmobilization and/or fusion of adjacent bones by implanting artificialassemblies in or on the spinal column. The region of the back whichneeds to be immobilized, as well as the individual variations inanatomy, determine the appropriate surgical protocol and implantationassembly. With respect to the failure of the intervertebral disc, theinterbody fusion cage has generated substantial interest because it canbe implanted laparoscopically into the anterior of the spine, thusreducing operating room time, patient recovery time, and scarification.

[0006] Referring now to FIGS. 1 and 2, in which a side perspective viewof an intervertebral body cage and an anterior perspective view of apost implantation spinal column are shown, respectively, a more completedescription of these devices of the prior art is herein provided. Thesecages 10 generally comprise tubular metal body 12 having an externalsurface threading 14. They are inserted transverse to the axis of thespine 16, into preformed cylindrical holes at the junction of adjacentvertebral bodies (in FIG. 2 the pair of cages 10 are inserted betweenthe fifth lumbar vertebra (L5) and the top of the sacrum (S1). Two cages10 are generally inserted side by side with the external threading 14tapping into the lower surface of the vertebral bone above (L5), and theupper surface of the vertebral bone (S1) below. The cages 10 includeholes 18 through which the adjacent bones are to grow. Additionalmaterial, for example autogenous bone graft materials, may be insertedinto the hollow interior 20 of the cage 10 to incite or accelerate thegrowth of the bone into the cage. End caps (not shown) are oftenutilized to hold the bone graft material within the cage 10.

[0007] These cages of the prior art have enjoyed medical success inpromoting fusion and grossly approximating proper disc height. It is,however, important to note that the fusion of the adjacent bones is anincomplete solution to the underlying pathology as it does not cure theailment, but rather simply masks the pathology under a stabilizingbridge of bone. This bone fusion limits the overall flexibility of thespinal column and artificially constrains the normal motion of thepatient. This constraint can cause collateral injury to the patient'sspine as additional stresses of motion, normally borne by the now-fusedjoint, are transferred onto the nearby facet joints and intervertebraldiscs. It would therefore, be a considerable advance in the art toprovide an implant assembly which does not promote fusion, but, rather,which nearly completely mimics the biomechanical action of the naturaldisc cartilage, thereby permitting continued normal motion and stressdistribution.

[0008] It is, therefore, an object of the present invention to provide anew and novel intervertebral spacer which stabilizes the spine withoutpromoting a bone fusion across the intervertebral space.

[0009] It is further an object of the present invention to provide animplant device which stabilizes the spine while still permitting normalmotion.

[0010] It is further an object of the present invention to provide adevice for implantation into the intervertebral space which does notpromote the abnormal distribution of biomechanical stresses on thepatient's spine.

[0011] Other objects of the present invention not explicitly stated willbe set forth and will be more clearly understood in conjunction with thedescriptions of the preferred embodiments disclosed hereafter.

SUMMARY OF THE INVENTION

[0012] The preceding objects of the invention are achieved by thepresent invention which is a flexible intervertebral spacer devicecomprising a pair of spaced apart base plates, arranged in asubstantially parallel planar alignment (or slightly offset relative toone another in accordance with proper lordotic angulation) and coupledto one another by means of at least one spring mechanism. This at leastone spring mechanism provides a strong restoring force when acompressive load is applied to the plates, and may also permit limitedrotation of the two plates relative to one another. While there are awide variety of embodiments contemplated, one preferred embodiment isdescribed herein as representative of preferred types.

[0013] More particularly, with respect to the base plates, which arelargely similar in all embodiments, as the assembly is to be positionedbetween the facing surfaces of adjacent vertebral bodies, and as suchneed to have substantially flat external surfaces which seat against theopposing bone surfaces. Inasmuch as these bone surfaces are oftenconcave, it is anticipated that the opposing plates may be convex inaccordance with the average topology of the spinal anatomy. In addition,the plates are to mate with the bone surfaces in such a way as to notrotate relative thereto. (The plates rotate relative to one another, butnot with respect to the bone surfaces to which they are each in contactwith.) In order to prevent rotation of a plate relative to the bone, theupper and lower plates alternatively may each include outwardly directedspikes which penetrate the bone surface and mechanically hold the platesin place. However, it is more preferably anticipated that the platesshould include a porous coating into which the bone of the vertebralbody can grow. The most desirable upper and lower plate surface porousfeature is a deflectable wire mesh into which the bone can readily grow,and which mesh will deform to seat into the concave upper and lower bonefaces. (Note that this limited fusion of the bone to the base plate doesnot extend across the intervertebral space.) These features, while beingpreferred are not required.

[0014] Between the base plates, on the exterior of the device, there mayalso be included a circumferential wall which is resilient and whichsimply prevents vessels and tissues from entering within the interior ofthe device. This resilient wall may comprise a porous fabric or asemi-impermeable elastomeric material. Suitable tissue compatiblematerials meeting the simple mechanical requirements of flexibility anddurability are prevalent in a number of medical fields includingcardiovascular medicine, wherein such materials are utilized for venousand arterial wall repair, or for use with artificial valve replacements.Alternatively, suitable plastic materials are utilized in the surgicalrepair of gross damage to muscles and organs. Still further materialswhich could be utilized herein may be found in the field of orthopedicin conjunction with ligament and tendon repair. It is anticipated thatfuture developments in this area will produce materials which arecompatible for use with this invention, the breadth of which shall notbe limited by the choice of such a material. For the purposes of thisdescription, however, it shall be understood that such a circumferentialwall is unnecessary, and in some instances may be a hindrance, andthusly is not included in the specific embodiment set forth hereinbelow.

[0015] As introduced above, the internal structure of the presentinvention comprises a spring member, or other equivalent subassemblywhich provides a restoring force when compressed. It is desirable thatthe restoring forces be directed outward against the opposing plates,when a compressive load is applied to the plates. More particularly, therestoring force providing subassembly comprises a multi-pronged domedmetal arch spring which is secured to the lower plate and againstmovement therefrom at its lateral ends. The domed spring comprises aplurality of flat ends (three to five in the preferred embodiments)disposed at the distal ends of the multi-prongs and a curvate or domedcentral portion. The curvate central portion is comprises a dome, or ashell. Stated alternatively, entire spring mechanism comprises a domedpiece of metal which has curvate sections removed from thecircumferential edge such that the element comprised a contiguouscentral curvate portion and a plurality (equal to the number of curvatesections removed) of curvate prongs which extend outward and downward.The flat lateral pieces which extend out from these prongs are theportions which are secured to the upper plate.

[0016] The domed portions of the strips deflect under loading, butprovide a restoring force in opposition to the loading until they arepermitted to regain their original shape. The restoring force of thespring is proportional to the elastic properties of the material as wellas the length and arc of the curvate central portion of the strip (andthe length and width of the prongs). The elasticity of the metal, whichendures and counteracts the strain of the material, causes a deflectionin the height of the arch.

[0017] In the preferred embodiment, the peak of the domed arch (centralportion) further comprises a socket for flexibly coupling to a postmember on the interior surface of the opposing plate. This post couplesto the spring to form a ball and socket joint at the peak of the domedarch, which joint permits the plates to rotate relative to one another.This rotation may be constrained by the specific conformation of thejoint such that the plates are free to rotate through only a range ofangles.

[0018] More particularly, this embodiment comprises a pair of spacedapart base plates, one of which includes means for coupling the flatlateral ends of the domed spring thereto it (such as simple set screws).The other of the plates is similarly shaped, having a flat exteriorsurface (which may include a mesh or porous coating to permit bonyingrowth), but further includes a short central post portion which risesout of the interior face at a nearly perpendicular angle. The top ofthis short post portion includes a ball-shaped knob. The knob includes acentral threaded axial bore which receives a small set screw. Prior tothe insertion of the set screw, the ball-shaped head of the post candeflect radially inward (so that the ball-shaped knob contracts). Theinsertion of the set screw eliminates the capacity for this deflection.

[0019] As introduced above, a domed arch spring is mounted to thisball-shaped knob in such a way that it may rotate freely through a rangeof angles equivalent to the fraction of normal human spine rotation (tomimic normal disc rotation). In order to couple with the post, the stripspring includes an socket which accommodates the ball-shaped portion ofthe post. More particularly, the socket includes a curvate volume havinga substantially constant radius of curvature which is also substantiallyequivalent to the radius of the ball-shaped head of the post. Thedeflectability of the ball-shaped head of the post, prior to theinsertion of the set screw, permits the head to be inserted into theinterior volume at the center of the spring, and the washer to berotated into the proper lordotic angulation. Subsequent introduction ofthe set screw into the axial bore of the post flexibly retains the headin the socket of the spring. This assembly provides ample spring-likeperformance with respect to axial compressive loads, as well as longcycle life to mimic the axial biomechanical performance of the normalhuman intervertebral disc.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a side perspective view of an interbody fusion device ofthe prior art;

[0021]FIG. 2 is a front view of the anterior portion of the lumbo-sacralregion of a human spine, into which a pair of interbody fusion devicesof the type shown in FIG. 1 have been implanted;

[0022]FIGS. 3a and 3 b are top views of the upper and lower opposingplates of one embodiment of the present invention;

[0023]FIGS. 4a and 4 b are a top view and side cross section view of adomed spring and a domed spring mounted to a lower plate; and

[0024]FIG. 5 is a side cross-section view of a second embodiment of thepresent invention which utilizes the elements shown in FIGS. 3a, 3 b, 4a, and 4 b.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0025] While the present invention will be described more fullyhereinafter with reference to the accompanying drawings, in whichparticular embodiments and methods of implantation are shown, it is tobe understood at the outset that persons skilled in the art may modifythe invention herein described while achieving the functions and resultsof this invention. Accordingly, the descriptions which follow are to beunderstood as illustrative and exemplary of specific structures, aspectsand features within the broad scope of the present invention and not aslimiting of such broad scope. Like numbers refer to similar features oflike elements throughout.

[0026] Referring now to FIGS. 3a and 3 b, side cross-section views ofthe top and bottom plate members 100 a 100 b of a first embodiment ofthe present invention are shown. More particularly, in this embodiment,the upper and lower plates 100 a, 100 b are nearly identical. As thedevice is designed to be positioned between the facing surfaces ofadjacent vertebral bodies, the plates include substantially flat surfaceportions 102 a, 102 b which seat against the opposing bone surfaces. Inaddition, the plates are to mate with the bone surfaces in such a way asto not rotate relative thereto. It is, therefore, preferred that theplates should include a porous coating into which the bone of thevertebral body can grow. The most desirable upper and lower platesurface porous feature is a deflectable wire mesh into which the bonecan readily grow, and which mesh 104 a, 104 b (see FIGS. 4b and 5) willdeform to seat into the concave upper and lower bone faces. (Note thatthis limited fusion of the bone to the base plate does not extend acrossthe intervertebral space.)

[0027] Plate 100 a further includes a single set of threaded holes 111for receiving the set screws (shown in FIG. 4b) required to affix thelateral ends of the multi-pronged domed arch strip spring thereto it.

[0028] Referring also to FIGS. 4b and 5, plate 100 b has a similarshaped to the plates described above, i.e., having a flat exteriorsurface 102 b which is designed to seat against the exposed opposingbone face in an intervertebral space, but plate 100 b further includes ashort central post member 105 which rises out of the interior face 103at a nearly perpendicular angle. The top of this short post member 105includes a ball-shaped head 107. The head 107 includes a centralthreaded axial bore 109 which extends down the post 105. This threadedbore 109 is designed to receive a small set screw 101. Prior to theinsertion of the set screw 101, the ball-shaped head 107 of the post 105can deflect radially inward (so that the ball-shaped head contracts).The insertion of the set screw 101 eliminates the capacity for thisdeflection.

[0029] Referring now to FIG. 4a, the multi-pronged domed arch spring 130of this embodiment is shown in a top view. The spring 130 comprises acentral domed portion 131 which includes a central socket portion (morefully described hereinbelow with respect to FIG. 4b), a plurality ofcurvate prongs 133 which extend outwardly and downwardly from the outeredges of the central domed portion, and a corresponding plurality oflaterally extending tabs 135 joined at the lateral ends of the prongs133. More particularly, the entire domed portion of the spring 130 isformed of a single contiguous shell member which has had several curvateportions removed from the lateral edge thereof. These removed portionsare spaced about the circumference of the spring, leaving a series ofprong shaped extending members 133 extending downward and outward alongthe arc of the shell. The lateral tabs 135 extend outwardly from theends of the prongs 133 and have holes 137 therethrough for beingattached to the upper plate 100 a, with set screws 139.

[0030] Referring now to FIG. 4b, this multi-pronged domed spring 130further includes the additional feature of having an enlarged centralopening 132. This central opening 132 includes a curvate volume 134 forreceiving therein the ball-shaped head 107 of the post 105 of the lowerplate 100 b described above. More particularly, the curvate volume 134has a substantially constant radius of curvature which is alsosubstantially equivalent to the radius of the ball-shaped head 107 ofthe post 105.

[0031] Referring also to FIG. 5, in which the fully assembled secondembodiment of the present invention is shown, the combination andassembly of this embodiment is now provided. The deflectability of theball-shaped head 107 of the post 105, prior to the insertion of the setscrew 101, permits the head 107 to be inserted into the interior volume134 at the peak of the domed arch strip spring 130. Subsequentintroduction of the set screw 101 into the axial bore 109 of the post101 flexibly couples the head 107 to the spring 130 by virtue of thehead 107 not being compressible and removable from the central volume134, but the post 105 being polyaxially retained in the socket 134.Ideally the post head 107 is locked loosely enough within the centralvolume 134 of the spring 130 such that anatomically relevant rotation ofthe plates 100 a, 100 b remains viable. In alternative variation,however, it is possible to design the coupling such that the locking ofthe set screw 101 in the head 107 locks the assembly in one rotationalorientation, preventing free rotation of the plates relative to oneanother. A combined embodiment may be one in which the set screw 101 maybe selectively positioned in an unlocked (but still securing for thepurpose of retention) and a locked orientation.

[0032] While there has been described and illustrated embodiments of anintervertebral spacer device, it will be apparent to those skilled inthe art that variations and modifications are possible without deviatingfrom the broad spirit and principle of the present invention. Thepresent invention shall, therefore, not be limited solely to thespecific embodiments disclosed herein.

We claim:
 1. An intervertebral spacer device, comprising: first andsecond plate members, each having plate surfaces thereof, said platesbeing disposed in a spaced apart relationship such that inner ones ofsaid plate surfaces oppose one another, and external ones of said platesurfaces face in opposite directions; and at least one multi-prongeddomed spring restoring force providing element disposed between theinner surfaces of said first and second plate members, and disposed suchthat a compressive load applied to the external faces of said plates iscounteracted by said at least one restoring force providing element. 2.The device as set forth in claim 1, wherein at least one of saidexternal surfaces of said first and second plates comprises a porouscoating.
 3. The device as set forth in claim 1, wherein said secondplate further comprises a post structure rising off the inner surfacethereof, and which post structure includes a ball-shaped head.
 4. Thedevice as set forth in claim 3, wherein said post further includes athreaded bore which extends axially from said ball-shaped headdownwardly, and which bore receives therein a threaded set screw suchthat prior to insertion of the set screw therein, said bore permits theball-shaped head to compress radially inwardly, and such that after theinsertion of said set screw said ball-shaped head is not readilyradially compressible.
 5. The device as set forth in claim 4, whereinsaid multi-pronged domed spring further comprises a central openingwhich includes a curvate volume for receiving and holding therein saidball-shaped head.
 6. An intervertebral spacer device, comprising: firstand second plate members, each having plate surfaces thereof, saidplates being disposed in a spaced apart relationship such that innerones of said plate surfaces oppose one another, and external ones ofsaid plate surfaces face in opposite directions; said second platemember further including a post structure rising off the inner surfacethereof, and which post structure includes a ball-shaped head; and amulti-pronged domed spring, having a central peak portion, said peakportion including a central opening which includes a curvate volume forreceiving and holding therein said ball-shaped head, such that acompressive load applied to the external faces of said plates iscounteracted by the restoring force of said spring.
 7. The device as setforth in claim 6, wherein said post further comprises a threaded borewhich extends axially from said ball-shaped head downwardly, and whichbore receives therein a threaded set screw such that prior to insertionof the set screw therein, said bore permits the ball-shaped head tocompress radially inwardly, and such that after the insertion of saidset screw said ball-shaped head is not readily radially compressible.